1. Field of the Invention
The present invention relates generally to an electrophotographic image forming device. More particularly, the present invention relates to a method and apparatus for compensating for scanning skew formed on a photosensitive medium through an optical scanning operation.
2. Description of the Related Art
In general, an electrophotographic image forming device such as a laser printer or a digital copy machine, forms image data input from a computer or a scanner onto a printing medium such as printing paper through a series of image forming processes. The image forming processes of the electrophotographic image forming device include processes of charging, writing, developing, transferring and fusing.
The electrophotographic image forming device may be divided into a controller and an engine. The controller analyzes and stores image data sent from a computer to a memory of the printer, communicates with the engine so that the engine can perform the image forming, and then transmits the data stored in the memory in the form of serial data. The engine includes mechanical elements that print the image data transmitted from the controller onto the printing paper. In the case of a laser printer, the main elements of the engine include at least an organic photoconductive drum (hereinafter referred to as a “photoconductive drum”), an optical scanner, and a developer.
FIG. 1 is a view illustrating the construction of an engine of an electrophotographic image forming device. The image forming device 100 is a color image forming device that can print a color image.
Referring to FIG. 1, the color image forming device 100 is provided with a first developer 110 that contains yellow (Y) toner, a second developer 120 that contains cyan (C) toner, a third developer 130 that contains magenta (M) toner, and a fourth developer 140 that contains black (K) toner. The color image forming device 100 is also provided with two photoconductive drums 157 and 167, a pair of optical scanners 159 and 169, and an intermediate transfer belt 170. All of the above-described constituent elements are provided inside a case 101 of the device 100.
The photoconductive drums 157 and 167 are exposed to light that the optical scanners 159 and 169 will scan to form electrostatic latent images. The first photoconductive drum 157, that is the upper one between the pair of photoconductive drums, is charged by a first charging roller 155, and is adjacent to the first and second developers 110 and 120 so that it can receive yellow (Y) toner and cyan (C) toner from the first and second developers to develop an image. The second photoconductive drum 167, that is the lower one between the pair of photoconductive drums, is charged by a second charging roller 165, and is adjacent to the third and fourth developers 130 and 140 so that it can receive magenta (M) toner and black (K) toner from the third and fourth developers to develop an image.
The intermediate transfer belt 170 is rotatably supported by a belt driving roller 171 that is connected to a motor shaft (not illustrated), a transfer backup roller 172 that is preferably an idle roller, and first and second support rollers 173 and 174 that are also idle rollers. As illustrated, the transfer belt 170 and rotates clockwise. First and second transfer rollers 175 and 176, provided inside the intermediate transfer belt 170, are arranged opposite to the first and second photoconductive media 157 and 167, respectively, with the intermediate transfer belt 170 being interposed between the first and second transfer rollers and the first and second photoconductive media.
A third transfer roller 180 is provided under the transfer backup roller 172 and is arranged opposite to the transfer backup roller 172 with the intermediate transfer belt 170 being interposed between the third transfer roller and the transfer backup roller.
Additionally, the electrophotographic image forming device 100 is provided with a fuser 185 for fusing a color image transferred onto a printing paper P by heat and pressure. Also provided is a feeder cassette 105 for loading the printing papers P, a pickup roller 182 for picking up the printing papers from the feeder cassette 105 paper by paper, a sorter 184 for sorting and conveying the picked printing papers, and first to third discharge rollers 186, 187 and 188 for discharging the printing paper P on which the color image is printed to the outside of the case 101.
The color image forming device 100 forms a color image in a manner that it transfers yellow (Y), magenta (M), cyan (C) and black (K) images onto the intermediate transfer belt 170 by superimposition to form a color image on the intermediate transfer belt 170. The color image forming device 100 then transfers and fuses the color image onto the printing paper P.
If light corresponding to yellow (Y) image information is scanned from the first optical scanner 159 to the first photoconductive drum 157 that is charged with uniform potential, a part of the drum on which the light is scanned comes to have a reduced resistance, and this causes charges attached to the outer peripheral surface of the first photoconductive drum 157 to escape from the outer peripheral surface of the drum 157. Accordingly, a potential difference occurs between the scanned part and the remaining part, and this causes an electrostatic latent image to be formed on the outer peripheral surface of the first photoconductive drum 157 being rotated. In this case, a yellow (Y) electrostatic latent image is developed as the yellow (Y) toner is supplied from the first developer 110 to the first photoconductive drum 157, and then a yellow (Y) image is transferred to the intermediate transfer belt 170 as the first photoconductive drum 157 is rotating.
Additionally, on the intermediate transfer belt 170, a magenta (M) image from the second photoconductive drum 167 is transferred and superimposed in the same manner as the transfer of the yellow (Y) image. After one-period of circulation on the intermediate transfer belt 170, a cyan (C) image from the first photoconductive drum 157 and a black (K) image from the second photoconductive drum 167 are transferred and superimposed in turn to form a color image.
Meanwhile, the printing papers P, loaded in the feeder cassette 182, are picked up by the pickup roller 182 for sorting by the sorter 184, and then pass through the third transfer roller 180 and the intermediate transfer belt 170. Thus, the color image is transferred onto the printing paper P. The color image transferred onto the printing paper P is then fused on the printing paper P by heat and pressure applied from the fuser 185, and the printing paper P on which the color image is fused is discharged to a discharge tray 102 provided outside the case 101 by the discharge rollers 186, 187 and 188.
FIG. 2 is a perspective view schematically illustrating the structure of the first optical scanner 159. FIG. 3 is a view illustrating side and center feeding of printing paper.
Referring to FIG. 2, the first optical scanner 159 is composed of a laser diode 200, a polygon mirror 204, a driver 202 and a reflecting mirror 206.
The laser diode 200 emits light. The driver 202 is a motor for rotating the polygon mirror 204 at a constant speed. The polygon mirror 204 scans the linear light irradiated from the laser diode 200 corresponding to the image signal. The reflecting mirror 206 reflects an incident light in a specified direction so that the reflected light is incident to the surface of the first photoconductive drum 157 on which the image is formed. Meanwhile, the second optical scanner 169 has substantially the same construction as the first optical scanner 159 as described above. Accordingly, a detailed description thereof is omitted for clarity and conciseness.
As the polygon mirror 204 rotates, the light emitted from the laser diode 200 is incident to an area drawn from a point “a” to a point “b” on the first photoconductive drum 157. Hereinafter, the area configured from the point “a” to the point “b” is called an optical scanning area. Generally, in performing the image printing work, the electrophotographic image forming device does not use the whole optical scanning area of the photoconductive area, but, uses only a reduced part thereof. This will now be explained with reference to FIG. 3.
The paper feeding process is classified into a center feeding process and a side feeding process. The center feeding process makes a center part of the printing paper pass through a center part c of the optical scanning area, and the side feeding process makes the printing paper pass through the optical scanning area as the printing paper slants to the left. In FIG. 3, the printing paper P1 indicates the printing paper fed by the center feeding process, and the printing paper P2 indicates the printing paper fed by the side feeding process.
An inclination of a scanning line formed in the optical scanning area of the photoconductive drum due to an optical scanning of the optical scanner is defined as a skew. This skew may occur due to a dimensional error of the optical scanner or the photoconductive drum. In forming the color image, four colors are superimposed. If directions and degrees of skews of plural scanning lines are different from one another, although the scanning lines have the skews that are within an allowable error range, the color image formed by the image superimposition may deteriorate in quality. Accordingly, skew compensation that makes the skews of the scanning lines of the respective colors coincide with one another in a specified allowable error range is required.
FIGS. 4A to 4C are views explaining a conventional method for compensating for a scanning skew performed in the color image forming device of FIG. 1. Since the color image forming device 100 illustrated in FIG. 1 is provided with a pair of optical scanners 159 and 169 and a pair of photoconductive drums 157 and 167, the skew of the scanning line scanned by one optical scanner is compensated for on the basis of the scanning line scanned by the other optical scanner. Hereinafter, it is assumed that the scanning line formed on the second photoconductive drum 167, by the optical scanning of the second optical scanner 169 (hereinafter referred to as a “second scanning line”), is compensated for on the basis of the scanning line formed on the first photoconductive drum 157 by the optical scanning of the first optical scanner 159.
Referring to FIG. 4A, although the first scanning line and the second scanning line should be superimposed without any skew, a skew of one dot occurs through the optical scanning area. In order to compensate for the one-dot skew, the optical scanning area is divided into two, and the second optical scanner 169 emits light that ascends by one dot in the right area. Referring to FIG. 4B, a skew of two dots occurs between the first scanning line and the second scanning line. In order to compensate for this, the optical scanning area is divided into three, and the second optical scanner 169 emits light that ascends by one dot in the second area and emits light that ascends by two dots in the third area. Referring to FIG. 4C, a skew of three dots occurs between the first scanning line and the second scanning line. In order to compensate for this, the optical scanning area is divided into four, and the second optical scanner 169 emits light that ascends by one dot in the second area, emits light that ascends by two dots in the third area, and emits light that ascends by three dots in the fourth area.
The conventional method for compensating for scanning skew as described above; however, has problems in that defects of the printed image due to discontinuation of the second scanning line on the boundaries of the divided optical scanning areas are relatively easily visible to the human eye. Particularly, if the skew of odd-numbered dots occurs with respect to the printing paper fed by the center feeding process, the discontinuation of the scanning line appears on the center part of the printing paper, while if the skew of even-numbered dots occurs with respect to the printing paper fed by the side feeding process, the discontinuation of the scanning line also appears on the center part of the printing paper. Thus, print defects that are much more noticeable result.
Accordingly, there is a need for an improved electrophotographic color image forming device which compensates for scanning skew formed in an electrophotographic color image forming device.